This invention relates in general to an automatic computer controlled technique for matching a set of points in a reference file against a set of points in a subject file wherein each of the points in the reference file and the subject file have values associated with certain predetermined parameters. More particularly, this invention relates to a fingerprint matcher in which the positional and angular parameters of a set of minutia points in a reference file are compared against the positional and angular parameters of a set of minutia points in a subject file to determine whether or not the subject file sufficiently matches the reference file so that the subject can be verified.
Fingerprint matcher mechanisms are known. One such matcher is described in Wegstein, J. H., The M-40 Fingerprint Matcher, National Bureau of Standards Technical Note 878, U.S. Government Printing Office, Washington, DC 1975. The matcher mechanism described in the Wegstein reference employs the position of each reference minutia and subject minutia in X and Y coordinates and the angular value in degrees. As described in Wegstein, each reference is compared with each subject point, within certain limits, for each of these X, Y and angle parameters so as to provide displacement values for a difference table. Certain clusters of points on the difference table are used to provide an indication of the extent to which the subject fingerprint might be displaced or rotated relative to the fingerprint from which the reference file was obtained. Certain figures of merit or matching scores are obtained. Only if the matching score exceeds certain thresholds, is the subject fingerprint deemed to correspond with the reference fingerprint.
The utility of a matcher is a function of the speed with which it operates and its error rate. There are two types of error. A type I error is the failure to verify a match when the subject finger is indeed the same as the finger from which the reference file was obtained. This is an incorrect rejection in an access system. A type II error is the verification of a subject fingerprint which in fact is not the same as the fingerprint from which the reference file was made. This is an incorrect admission in an access system.
The value of any matcher mechanism is the extent to which the matcher contributes to increasing the speed of operation of a verification system, to reducing type I errors and to reducing type II errors.
There is a trade off between these three operating characteristics and the amount of distortion and lack of registration between the two fingerprints that are being compared which can be tolerated. A major, if not the major, problem in matching fingerprint derived data is due to the plasticity of the finger. The resultant variation in finger impression from time to time results in substantial image variation.
Accordingly, it is a purpose of this invention to provide an enhanced matcher in which an improved trade off is obtained between the characteristics of speed, type I error, type II error, and ability to accept distortion and lack of registration.